My daughter was diagnosed in 2007. Fall of 08 we did the genetic testing....which I believe only tests the most common 100 (there are over 1600 mutations in the database now). NO mutations were found. She had already had a positive sweat test, and showed significant signs in her bowels. They then sent the blood work on to the lab in Toronto and fan a full panal, looking at the entire CFTR gene. This did take a long time....not like the other Canadian indicated...our results took about 4 months, which is understandable since it is very extensive. Anyways, they found one mutation that was very rare...only in database a handful of time in comparison, and the other mutation was one that had never been recorded before, it is a deletion mutation on exons 23 and 24. New mutations are being discovered all of the time. We are fortunate here that if there is a CF diagnosis they will do the full panal in order to narrow down the exact mutation. But we would have been in a similar situation to you, if sweat test was negative, there would have been no reason to do the full panal. Doe your son have any other CF symptoms? Salty tasting skin...abnormal stools and or appetite?